Nuclear Security involves knowledge of the presence of nuclear explosives, materials that can be employed in their fabrication, and materials that can be employed in what has become known as a “dirty bomb” where radioactive material is combined with conventional explosives which can spread radioactive material over a significant area. To this end it is important to improve portal monitoring for detection of illicit nuclear material in cargo containers. Radiation portal monitors (RPMs) are designed to be used at road, rail, airport or seaport checkpoints to detect gamma and neutron radiation from radioactive and nuclear materials. RPM are a preferred option where the traffic of goods can be funneled into narrow confines, typically referred to as nodal or choke points. The sensitivity of detectors needs to be high and the detection data needs to be collected rapidly and analyzed in a sufficiently rapid manner that commerce is not seriously impeded. The monitoring device must be suited to the application and positioned for collection of data exclusively from the container of interest. In addition to detection of radiation, it is desirable to identify the radio isotope source of the radiation. X-ray techniques do not readily distinguish between fissionable nuclear materials and innocuous high-Z materials like lead or tungsten that are legitimate cargo. Gamma spectroscopy is not always effective since many materials of interest are not highly radioactive and are easily shielded. Yet, typical identification is carried out by measurement of the gamma spectrum to identify a radionuclide. This is complicated if the radiation source is a mixture of radionuclides. Identification requires an effective algorithm to analyze the convoluted gamma spectrum, as reviewed in Burr et al., Algorithms 2009, 2, 339-360. Identification is essential to categorization of the event and determination of the appropriate response to the event. Appropriate isotopes must have half-lives, on the order of hours, and intermediate energies, between 250 and 1400 keV, and a measurement time of approximately one day is required to produce reaction rate uncertainties on the order of 2%.
There are many challenges to be overcome in the screening for threat isotopes; not the least of which is that there are approximately 200 radioisotopes. The radioisotopes can belong to: medical isotopes, such as 67Ga, 51Cr, 75Se, 99mTc, 103Pd, 111In, 123I, 125I, 131I, 201Tl, and 133Xe; industrial isotopes, such as 57Co, 60Co, 133Ba, 137Cs, 192Ir, 204Tl, 226Ra, and 241Am; naturally occurring radioactive material (NORM) isotopes, such as 40K, 226Ra, 232Th and its daughters, and 238U and its daughters; and special nuclear material (SNM) isotopes, such as 233U, 235U, 237Np, and Pu isotopes. There is no accurate figure concerning the number of radioactive material sources throughout the world. In addition to power plants, where the fuels and wastes contain Uranium 235 and Plutonium 239 among other isotopes, there are many uses that employ the radioisotopes Cobalt 60, Strontium 90, Cesium 137, and Iridium 192, and the sheer number of applications for these materials make them inherently difficult to track and control. Many of these materials are lost, stolen, or simply abandoned when no longer required; for example, an average of about 300 sources of radioactive material are reported lost or stolen each year in the United States. These “orphaned” radioactive sources are an immense concern and even more troublesome are sources from countries where civil authority and regulatory oversight are weaker.
Scanning rail cargo and other broadly distributed moving containers is a significant challenge with many differences from standard cargo container scanning that can be carried out with stationary containers. This screening requires rapidly determining the presence of nuclear material in a moving rail car. Methods that can readily perform such screening generally require the disruption of commerce.
Another problem involves the waste stream assessment and environmental processing systems employed during the decommissioning of nuclear facilities, power plants, and weapons complexes where materials must be assessed for contamination. An effective and rapid assessment method to distinguish non-contaminated from contaminated material, as well as determining fissile contamination across large environmental areas is needed.
Spent fuel assay, particularly plutonium assay and cask storage verification, is an important goal in nuclear safeguards to verify quantities of fissile material in spent fuel to ensure that no material has been illicitly diverted for the production of weapons. Current spent fuel safeguards techniques are passive, showing only that the spent fuel has not been removed and do not characterize the spent fuel. Current techniques rely on computer codes and passive measurements of the spent fuel. Current Pu assay techniques are not able to accurately determine the Pu mass in spent fuel. Several reports have concluded that a combination of assay techniques is needed to get the desired accuracy. More complete and accurate analysis of the spent fuel would improve the safety margin and arrangement of spent fuel in dry storage by allowing the determination of the number and arrangement of spent fuel in a dry cask when transferring fuel from pool to dry storage. An improved method of analysis could explicitly identify and characterize spent fuel bundles to discourage fuel pin/bundle diversion. Direct measurements made from an active interrogation of the spent fuel to determine the composition of the fuel to permit recording of the bundle's signature would be useful. Such an assessment of spent nuclear fuel would aid in criticality calculations and potentially reduce the cost of dry storage of spent fuels or help ensure that no nuclear material is diverted during a reprocessing of the spent fuel. A system that could be integrated with other techniques to improve and assure the accuracy of the assay and determine Pu content is needed.
Nuclear fuel enrichment facilities require non-destructive monitoring of enrichment and flow for both process monitoring and treaty/safeguards compliance. Non-destructive assay (NDA) of spent nuclear fuel with a direct and independent determination of plutonium (Pu) mass in spent fuel is increasingly important for international safeguards.
Rather than employing passive detection, active approaches to detection have been promoted by using probing beams, such as a neutron beam. The nuclear material of fuels can be probed by irradiation in a test reactor or by irradiation using an external neutron source. After irradiation the gamma spectrum is measured and specific gamma lines are correlated to the induced reaction rates. Chosen fission products must have half-lives on the order of hours and intermediate energies, between 250 and 1400 keV. This method requires measurement time of about one day to produce reaction rate uncertainties on the order of 2%. Unfortunately, the high irradiation background from spent fuel prevents using of the conventional gamma-scanning method to measure its fission rate.
A high-energy delayed gamma technique developed for spent fuel involves modifying the existing gamma-scanning method by using high-energy lines from delayed gamma precursors (above 2 MeV), which can be measured above the background after neutron interrogation. This technique is hampered by the poor-quality nuclear data associated with the high energy gamma lines. Even with measurements over several hours, the random uncertainty on induced fission rates is several percent and there are systematics contributions that also add several percent to the uncertainty.
An alternative is a technique that uses delayed neutron measurements. Delayed fission neutrons are induced by the interrogator source, which are measured with the passive neutron emission treated as a background, although doing so increases the statistical uncertainty of the measurement. Typically, 1-2 minutes of measurement data can be obtained for each 15 minute irradiation period, which is the time required for saturation of delayed neutron precursors, with the achievement of random uncertainties of 2% for a series of measurements. Systematic uncertainties can contribute up to 5% additional uncertainty.
Discrimination between probing neutrons and fission induced prompt neutrons is often very difficult, particularly when the energy of the probing neutrons is similar in energy to that of the more energetic prompt neutrons generated or when large containers are involved. Alternative techniques induce fission events with pulsed external neutron sources and detect the much weaker delayed neutron emission from fission products to distinguish the induced signal from the probing neutrons. More recently, Betozzi et al., U.S. Pat. No. 8,358,730 teaches a method to identify actinide nuclear materials by analyzing energetic prompt neutrons after neutron induced fission with lower energy incident neutron where a plurality of detectors is placed at different angles relative to the incident neutron beam. The method could identify an actinide by the energy distribution change upon irradiation with a second incident neutron. There is no apparent teaching of discerning the composition of a mixture of fissionable materials.
Hence, there remains a need for a relatively rapid method to inventory the components of a radioactive material for control of nuclear materials.